Composite body of magnesium and aluminum and method of making same



P" 20, 1955 M. R. BOTHWELL 3,179,504

COMPOSITE BODY OF MAGNESIUM AND ALUMINUM AND METHOD OF MAKING SAME Filed March 27, 1964 INVENTOR Marv/r2 R. 50//;we//

ATTORNEYS United States Patent COMPOSITE BODY OF MAGNESIUM AND ALU- MINUM AND METHOD OF MAKING SAME Marvin R. Bothweil, Midland, Mich assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Mar. 27, 1964, Ser. No. 356,355 3 Claims. (Cl. 29-1975) This invention relates to an improved composite body of magnesium and aluminum and is particularly concerned with production of a composite body consisting of a core of a magnesium or magnesium-base alloy containing at least 75 percent of magnesium (referred to herein as a magnesium metal) provided on one or more of its surfaces with a coating of aluminum or aluminum-base alloy containing at least 75 percent of aluminum (referred to herein as an aluminum metal) and characterized by a greatly improved resistance to galvanic corrosion.

This is a continuation-impart of my copending application Serial No. 841,430, filed September 3, 1959 In my copendingj application Serial No. 837,940, filed September 3, 1957, now US. Patent 3,052,331, is described a method of suppressing bimetallic couple corrosion of a magnesium metal article in a composite structure. The method consists of interposing a barrier layer, or separating means, such as a cladding applied to the magnesium metal article, and formed of a galvanically compatible aluminum metal, between the magnesium metal article and each galvanically incompatible metal article in the structure.

For the purposes of the specification and claims such terms as galvanically compatible or galvanically incompatible are to be understood to be' with reference to magnesium metal.

The protection from corrosion of a magnesium metal core by a coating of an aluminum. metal has hereto-fore presentedvgrave difficulties. A coating of commercially pure aluminum (99.0 to 99.9 percent aluminum) or aluminum-base alloy exhibits high resistance to corrosion and provides good protection to a magnesium metal core while present over it as an impervious physical barrier. But if the core becomes locally exposed, as in the instance that a composite body of the type described in the form of a stock shape such as a sheet, is sheared during fabrication into finished goods such as structural parts, thereby exposing the core at the sheared edge, or if other physical or chemical damage occurs to this barrier, as in mechanical handling of a casting clad on all surfaces with an aluminum metal coating, preferential corrosion of the core at the exposed site is extremely rapid because of the galvanic action between the coating and the core. This preferential attack on the core metal occurs when the core metal has a more active solution potential than that of the coating metal whereby galvanic action may be set up between the two metals in contact with a corroding medium thus causing a flow of current which tends to dissolve the core metal. The effect of the galvanic action is heightened if the action is focused on relatively small exposed areas rather than diffused over a large surface. The resultant damage to the core takes place more rapidly than it would in the absence of contact with a galvanically incompatible aluminum metal coating. In addition, the problem is compounded due to the fact that the aluminum metal coating is itself readily attacked by the alkaline products of the corrosion of the magnesium metal core.

It is the object of this invention to provide a composite body consisting of a magnesium metal core provided with a discontinuous aluminum metal coating and exhibiting improved corrosion resistance.

Another object of this invention is to provide a composite body consisting of a magnesium metal core having an aluminum metal coating which exhibits good galvanic compatibility with exposed portions of the core thus minimizing galvanic corrosion of exposed portions of the core.

Another object of this invention is to provide a method for the production of a composite body of improved corr'osion resistance consisting of a magnesium metal core having less than complete enclosure by an aluminum metal coating which shows good galvanic compatibility with exposed portions of the magnesium metal core.

Other objects and advantages of the invention will become apparent to those skilled in the art upon becoming familiar with the following description and claims.

This invention is predicated on the discovery that by rigorously controlling the purity of an aluminummetal applied as a coating to a magnesium metal core, the galvanic compatibility of the aluminum metal coating with an eX- posed portion of the core is enhanced whereby the life of the resulting composite body in a corrosive environment is increased.

The criterion for high galvanic compatibility is that the coating should polarize to the solution potential of the core at a very low cathodic current density, since this current determines the rate of galvanic corrosion of the core. Normally active magnesium metal will not be sacrificial to the more noble metal aluminum it contacts if galvanic action is soon arrested by polarization of the aluminum metal surface. If cathodic current flow substantially stops, the corrosion rate of exposed portions of a magnesium metal core approaches that of the unclad magnesium core metal itself.

Polarizability of the surface of an aluminum metal is closely related to the concentration levels therein of certain critical alloying metals and metal impurities. Common impurities in commercial grade aluminum are iron, copper, and silicon. It has been found that polarization of aluminum metal Will more readily occur if the iron content of the aluminum metal is below 0.05 percent by weight, the copper content below 0.1 percent,-the nickel content below 0.1 percent and the combinedtotal of iron plus copper plus nickel content below 0.1 percent. It is to be preferred that the aluminum metal contain less than 0.05 percent combined total iron plus copper plus nickel content. The total combined content of lead, bismuth, chromium and titanium should also be below about 0.5 percent. On the other hand, up to 1 percent manganese, 15 percent magnesium and 3 percent of silicon, singly or in combination, do not adversely affect the polarizability of aluminum metal when alloyed therewith. The presence of zinc is less markedly detrimental than the pressential that the aluminum metal employed in the cornposite body of the invention have the property of polarizing in a corrosive environment to substantially the solution potential of the magnesium metal core in contact therewith upon the passage of a small cathodic current. For example, on immersing the composite body of the invention, having an exposed core portion, in a 3 percent aqueous solution of NaCl which is saturated with respect to Mg(OH) the aluminum should polarize upon being subjected to'a cathodic current density of less than 0.5 milliampere per square inch of cathode area and preferably less than 02 milliampere per square inch of cathode area.

According to one method of forming a composite body, the surface of a magnesium metal core in rolled form to be coated is cleaned as by wire brushing then placed between two clean galvanically compatible aluminum cycling.

metal sheets. The aluminum metal sheets may be welded to the magnesium metal core, if desired, to retain juxtaposition of the sheets with the core. The composite is then heated to a temperature between 300 and 900 F., bonded together by rolling and further worked as by rolling, extruding or forging. An aluminum metal coating may also be applied to a magnesium metal core according to various additional known methods as follows: (1) by bonding with adhesives, (2) by application in a molten or semimolten condition by dipping or spraying, (3) by electrodeposition, (4) by deposition from a vapor, (5) by the reduction or decomposition of aluminum-bearing compounds and (6) by impacting with finely divided aluminum metal powder, i.e., mechanical plating. It will be recognized by one skilled in the art that the composition of the aluminum metal coating obtained by the second, third and fourth coating methods listed above is not necessarily identical in composition to the aluminum metal used as starting material. While possibly more difficult to control, these methods have the advantage that they permit the coating of irregularly shaped cores, or other cores not in flat rolled form.

The composite body of my invention may be provided on any one or all surfaces with a protective coating of a galvanically compatible aluminum metal. The superior corrosion resistance of the composite body, of course, as compared to the composite body as previously known, is not demonstrated unless at least a portion of the magnesium metal core is exposed to a corrosive environment.

Examples of the composite body of my invention are shown in FIGS. 1 and 2 of the appended drawing.

FIG. 1 shows a composite body consisting of a core of a magnesium metal 1 provided on its top and bottom surfaces with a coating 2 and 3, respectively, of galvanically compatible aluminum metal. In FIG. 2 is shown in transverse cross section a composite body having a core of a magnesium metal 5 of generally pi-shaped cross section provided on all sides with a coating 4 of galvanically compatible aluminum metal.

FIG. 2 illustrates a type of composite body which is useful as a corrosion resistant structural member but which has exposed surfaces not readily coated by a rollbonding method.

The following is an example of the preparation of a composite body of the type herein disclosed and claimed. Very high purity aluminum (nominally 99.99 percent purity) was melted in a pure graphite crucible and cast in a graphite mold to give a 1" x 4" X 4 rolling slab. The slab was surfaced with abrasive paper and then rolled to a sheet 0.022 inch thick. This sheet so rolled was wire brushed and then a piece of the sheet was placed over each side of 0.25 inch AZ3 lA magnesium alloy sheet, the sides of which had also been wire-brushed. This composite was heated to about 700 F, bonded together by rolling,

regarded as standard and known as the alternate immersion method. This method as applied to magnesium alloys comprises ascertaining the weight loss resulting from immersing a weighed sample of the material of measured area in a 3 percent aqueous sodium chloride solution at room temperature for 2 minutes, withdrawing the sample,

holding it in the air for 1 minute, and then repeating this cycle for a protracted period. The corrosion rate is obtained by computing the loss in weight of the sample per square centimeter of surface area per day of such it has been found that sea water does not become nearly as alkaline as 3 percent sodium chloride solution during the corrosion of aluminum-magnesium composites because of the buffering action of magnesium salts contained in .sea water. Accordingly, test solutions ,of sodium chloride as well as sea water have been used.

In corrosion tests reported below of composites made as described, corrosion resistance values in 3 percent sodium chloride and in sea water are expressed in terms of the average weight loss per day per unit area over various test periods. For the test, sample coupons were sheared from composite sheets prepared in accordance with the invention, by coating magnesium alloy AZ31A sheet with various aluminum metals as listed in the tables. These coatings were applied by rolling the composites at about 700 F. Blanks as noted in the table for comparison were similarly prepared with other alloys.

The results in Table 1 illustrate the deleterious etfect of iron contamination in aluminum metal coatings for these composites.

Similarly, the results in Table 2 illustrate the deleterious effect of copper contamination.

The results in Table 3 illustrate the less critical nature of zinc contained in aluminum metal coatings for these composites.

TABLE 1 Efiect of iron ((Jorrtjsiou get? mg. cm a Per- Pery Test No. Al Alloy Type cent cent Fe Si 1 Day 1 Week 3% Sea Water NaCl Pure B 0.004 0.006 0.29 0.043 (2 wks.)

03 005 3. 1 1.2 .025 .04; 2.6 1.2 03 027 1. 5 60 074 039 6. 5 2.2 41 050 21. 6 4.6 .47 .17 18. 9 3.1 (2 days) A=High purity aluminum, 99.9 percent Purity Grade. B =Supcr purity aluminum 93.99 percent Purity Grade. 0 O =C ornmercial purity aluminum, 28 alloy.

TABLE 2 Efiect of copper glorrtlislon lgatge m cm. a Al Alloy Por- Pen Pen g y Text No. Type ccnt ccnt cent Cu Fe Si 1 Day 1 Week 3% Sea Water N aOl 1 Purc B--- 0. 004 0. 004 0. 006 0. 29 0.043

(2 wks.) B-I-Cu..- 03 01 005 11. 2 .037 B-f-Ou. 0S 01 005 13. 7 .067 Pure A... 003 03 027 1.5 .60 A+ Cu 2 035 036 36 3.5 A-l-Cu. 2. 1 037 038 42 Destroyed A=High purity aluminum 99.9 percent Purity Grade. b B =Super purity aluminim 90.9 percent Purity Grade.

TABLE 3 Effect of zinc glorrclision/(litatf mg. cm!- a Al Alloy Icr- Per- Pery Test No. Type cent cent cent Zn Fe Si 1 Day 1 Week 3% Sea NaCl Water Pure A--- 0. 01 0.030 0. 027 1. 5 060 A-|-Zn 16 .030 037 3. 0 28 A-l-Zn 2. 05 O34 037 8. 0 46 u A=High purity aluminum 99.9 percent Purity Grade.

I claim:

1. An improved structural member comprising a composite body consisting of a core of a magnesium metal having metallurgically bonded thereto on at least one surface thereof a coating of an aluminum metal, said aluminum metal containing less than by weight 0.05 percent of iron, 0.1 percent of copper, 0.1 percent of nickel and less than the combined total of 0.1 percent ironplus copper plus nickel, up to 1 percent of manganese, up to 15 percent of magnesium, up to 3 percent of silicon, and a proportion of zinc up to 2.5 percent plus 0.5 times the magnesium content in Weight percent and said aluminum metal having the characteristic, that when subjected to a corrosive environment in the presence of the magnesium metal, of being polarized when subject to a cathodic current density of less than 0.5 milliampere per square inch of cathode area and assuming substantially the solution potential of the magnesium metal.

2. The improved structural member as in claim 1 in which the coating of aluminum metal contains a combined total iron plus copper plus nickel content of less than 0.05 Weight percent.

3. An improved structural member comprising a composite body consisting of a core of a magnesium metal having metallurgically bonded thereto on at least one surface thereof a coating of an aluminum metal, said aluminum metal consisting essentially of at least 75 perbelow about 0.5 percent, up to 1 percent of manganese, up to 15 percent of magnesium, up to 3 percent of silicon, and a proportion of zinc up to 2.5 percent plus 0.5 times the magnesium content in Weight percent and said aluminum metal having the characteristic, that When suhiected to a corrosive environment in the presence of the magnesium metal, of being polarized when subject to a cathorlic current density of less than 0.5 milliampere per square inch of cathode area and assuming substantially the solution potential of the magnesium metal.

Reierences Cite by the Examiner UNITED STATES PATENTS 2,054,054 9/36 Iarman 29197.5 2,100,257 11/37 Larson 29-1975 2,721,172 10/55 Higgins 204-197 OTHER REFERENCES Anodic Protection, by Edeleany and Gibson, Chemistry and Industry, March 11, 1961, pages 301307.

Corrosion Control by Anodic Protection, by Edeleany M.A.-Phd. September 1954-, Metallurgia copy.

DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, Examiner. 

1. AN IMPROVED STRUCTURAL MEMBER COMPRISING A COMPOSITE BODY CONSISTING OF A CORE OF A MAGNESIUM METAL HAVING METALLURGICALLY BONDED THERETO ON AT LEAST ONE SURFACE THEREOF A COATING OF AN ALUMINUM METAL, SAID ALUMINUM METAL CONTAINING LESS THAN BY WEIGHT 0.05 PERCENT OF IRON, 0.1 PERCENT OF COPPER, 0.1 PERCENT OF NICKEL AND LESS THAN THE COMBINED TOTAL OF 0.1 PERCENT IRON PLUS COPPER PLUS NICKEL, UP TO 1 PERCENT OF MANGANESE, UP TO 15 PERCENT OF MAGNESIUM, UP TO 3 PERCENT OF SILICON, AND A PROPORTION OF ZINC UP TO 2.5 PERCENT PLUS 0.5 TIMES THE MAGNESIUM CONTENT IN WEIGHT PERCENT AND SAID ALUMINUM METAL HAVING THE CHARACTERISTIC, THAT WHEN SUBJECTED TO A CORROSIVE ENVIRONMENT IN THE PRESENCE OF THE MAGNESIUM METAL, OF BEING POLARIZED WHEN SUBJECT TO A CATHODIC CURRENT DENSITY OF LESS THAN 0.5 MILLAMPERE PER SQUARE INCH OF CATHODE AREA AND ASSUMING SUBSTANTIALLY THE SOLUTION POTENTIAL OF THE MAGNESIUM METAL. 